Process for producing cement from cement slurry and a plant for carrying out the process

ABSTRACT

Cement is produced from cement slurry which is spray-dried and preheated by means of exhaust gas from a kiln. The heat exchange between the exhaust gas from the kiln and the spray-dried material is controlled observing certain temperature limits, whereby an efficient elimination of harmful alkalies is obtained. Saving of investment is possible due to the fact that the kiln can be constructed shorter, or the capacity of an existing kiln can be increased.

United States Patent Gude et al. 1 June 13, 1972 PROCESS FOR PRODUCINGCEMENT References Cit d FROM CEMENT SLURRY AND A UNITED STATES PATENTSPLANT FOR CARRYING OUT THE 3,317,201 5/1967 Muller et al. ..263/32PROCESS 2,819,172 1/1958 Trief ..263/5 3x FOREIGN PATENTS ORAPPLICATIONS [72] Inventors: Klaus Erik Gude, V1rum; Bjorn Lund,

Kobenhavn F, both of Denmark 755,029 1/1954 Great Britain ..263/32 [731Assignee: Aktieselskabet Niro Atomizer, Soborg, Primary ExamineFJohn J.Camby Denmark Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak [22]Filed: Sept. 4, 1970 21 Appl. No.: 69,801 [57] ABSIRACT Cement isproduced from cement slurry which is spray-dried and preheated by meansof exhaust gas from a kiln. The heat [30] Foreign Apphcatmn Pnomy Damexchange between the exhaust gas from the kiln and the spray- Sept. 10,1969 Denmark ..4850/69 dried material is controlled observing certaintemperature limits, whereby an efficient elimination of harmful alkaliesis [52] Us. Cl ..263/32 R, 263/21 A obtained, Saving of investment ispossible due to the fact that l 1 CI "F271, the kiln can be constructedshorter, or the capacity of an exist- [58] Field Of Search ..263/2i A,32 R, 53 R ing kiln can be increased,

7 Claims, 1 Drawing figure ALKALI PROCESS FOR PRODUCING CEMENT FROMCEMENT SLURRY AND A PLANT FOR CARRYING OUT THE PROCESS The presentinvention relates to a process for producing cement from cement slurrywhich is spray-dried and preheated by utilizing exhaust gas from acalcining and burning kiln, as well as a plant for carrying out theprocess.

The starting materials employed in cement production contain, to agreater or lesser degree, compounds which, in the burning of thematerial to cement, give rise to the formation of vapors containingalkali metals, normally, in the main potassium, in the form of oxides,chlorides or sulphates or as other volatile compounds. In conformitywith the terminology used in literature on cement production, theserelatively volatile alkali metal compounds are designated as alkali" inthe present specification.

The presence, in the raw materials, of alkali or of compounds, which inthe heating process of the raw materials, particularly in the sinteringprocess of same, form alkali vapors, pose serious problems which, on theone hand, are due to the fact that a high alkali content in the finishedcement reduces the quality of same as a consequence of the fact that thealkalis may react with certain additives used in concrete production andas a consequence of the fact that the alkalis may affect the settingqualities of the cement and, on the other hand, that the alkalis giverise to difficulties in the cement burning process proper. For, in thisprocess, a so-called alkali cycle will be produced due to the alkalinevapors produced in the calcining and burning zone of the burning kilnbeing conducted into the kiln with the hot gas, which gas is later oncooled off by contact with material that has not been burned yet duringthe preheating of same. Irrespective of whether the cement production iseffected as a classical wet process, in which the slurry is introducedas such into a rotary kiln, or whether a plant for drying and preheatingthe slurry or raw meal is placed before the rotary kiln, an alkalicondensation will take place with the cooling of the gas which, in thefirst case, may deposit itself in the form of a ring in the kiln anddisturb its operation and, in the second case, may form flowimpedingdeposits in the preheating plant, particularly in cyclones which maypossibly be employed in same. In both cases, a serious upsetting of thebalance necessary for carrying out the cement production, may arise. ltfurthermore happens in this case that alkali settles on the raw materialwhich has not been sintered yet, which results in that, into thesintering zone, an additional amount of alkali to that contained in theraw material proper, is being introduced, whereby the problems mentionedare aggravated. Finally, it has to be stated that the said alkali cycleresults in a poorer economy, in that calories for the evaporation of thealkali by the high temperature in the sintering zone are consumed, whichcalories are only liberated by the condensation at the lower temperaturein the preheating process.

It consequently is particularly desirable to break this alkali cycle bya continuous removal of alkali from the system and various methods havebeen suggested for this purpose. It has thus been proposed in cementproduction by the dry process, to remove alkali from the system byomitting to utilize the total volume of hot gas from the kiln forpreheating the raw meal, and instead of it, to allow a part of same toescape into the atmosphere immediately upon removal from the kiln.Naturally, this is an expensive method for alkali removal since the heatin the gas that is purged, goes to waste. Moreover, the amount of alkaliwhich is allowed to escape into the atmosphere with the hot gases, couldgive rise to pollution of the environs of the cement factory. Moreover,in connection with the so-called Lepol kilns, in which the exhaust gasfrom a rotary kiln serves to preheat and dry the raw material, it isknown to carry out a removal of alkaline dust from the gas subsequent tosame having been used for preheating, but prior to it being employed fordrying.

It has, furthermore, been proposed to solve the problems connected withthe alkali by effecting the burning in two buming steps with separatefiring, so that the calcining is carried out in the first burning stepand the sintering in the second burning step, in which case only the gasissuing from the sintering contains alkali which is not recycled intothe system, by employing special, less effective heat exchangers whenutilizing the heat content of the gas for preheating the raw material.After the heat exchanging, the temperature of the gas has dropped tosuch an extent that it should be possible to remove its alkali contentin the form of dust and the gas is thereupon utilized further for dryingthe raw material, for instance, for spray-drying the slurry. Finally, incertain cases, it is known to carry out a wet working up of the materialaccumulated in the dust filters for removing alkalis from same prior tothe material being led back into the burning kiln.

However, it does hold good for these known methods that they result insignificantly increased plant or operational costs and that they are notin all cases able to provide a sufficiently effective alkali removal.

The invention is based on the recognition that a particularly effectiveremoval of alkali can be achieved involving low operational costs and ina plant requiring relatively modest plant expenditure by means of aprocess which, according to the invention, is characterized in that theexhaust gas issuing from the kiln is heat-exchanged with the spray-driedmaterial to such an extent that the temperature of the gas does not dropbelow approximately 750 C, while the spray-dried material is heated toat least 550 C, and in that the gas is subsequently used as drying gasin the spray drying, whereby it is cooled further to not above 300 C.

Be means of this process, a very effective utilization of the heatpresent in the exhaust gas from the kiln is achieved, while, at the sametime, removing the alkali content of the gas.

As a consequence of the temperatures selected in the heat exchanging,the preheated material issuing from the heat exchanger will not to anysignificant degree have received alkalis from the gas since, dependingupon the construction of the heat exchanger either no deposition ofalkali on the material will occur at all, or, if such a deposition doestake place, it is going to evaporate again when the temperature of thematerial rises in the heat exchanger. The main part of the amount ofalkali present in the exhaust gas of the kiln will thus be conductedfurther with the gas into the spray drying apparatus. In this, thealkaline gas having a temperature of at least 750 C, contacts theatomized aqueous slurry. By this means, an intensive evaporation fromthe atomized particles will take place and, at the same time, as aconsequence thereof, a cooling off of the gas, by means of which thealkali is separated. The cooling of the gas occurs quite close to thewet particles, but on account of the strong evaporation from same, thealkali is not deposited at all, or else only to a very slight degree, onthe particles of the atomized slurry, but remains in the gas in the fonnof very fine particles. Since the gas is cooled to a temperature of 300C or lower, these particles are present as a fine dust which is nottacky and which consequently does not tend to agglomerate or settle onthe walls of the atomizer. These fine alkaline particles aresubsequently led away from the spray drying apparatus with the exhaustgas, possibly together with a minor quantity of fine particles formed bythe atomization of the raw meal, to a suitable particle separationplant.

A preferred embodiment of the process is, according to the invention,characterized in that the heat exchanging is effected by dispersing thedried material in a gas stream and by letting it be transported into theheat exchanger, whereupon it is separated by means of a cyclone, wherebysuch volumes of gas and quantities of material and such temperatures areemployed that the temperature of the exhaust gas from the cyclone is atleast 800C, while the material is heated to at least 600C, and the gasis cooled to not above 200C during the spray drying. By means of thisembodiment, an effective heat exchanging by the use of a relativelycheap and reliable apparatus is rendered possible, while the statedtemperatures ensure that any possible alkali which, by this cocurrentheat exchanging, may settle on the relatively cold material when same isbrought into contact with the very hot gas from the kiln, will againvaporize prior to the material being taken out of the cyclone. In thisembodiment, the maximum temperature employed of the gas issuing from thespray drying apparatus, ensures, even under extreme conditions, that noalkali agglomeration takes place in the spray drier.

Furthermore, the invention relates to a plant for carrying out theprocess, which plant is characterized in that it comprises a heatexchanger of the kind which is able to equalize temperature differencesbetween gas and powdered material, as well as a spray drier, whereby theheat exchanger is provided with a gas feed opening for connection to thegas discharge aperture of a calcining and burning kiln, a gas dischargeaperture which is connected to the gas feed aperture of the spray dryingapparatus, and a supply aperture for powdered material which isconnected to the discharge aperture of the spray drying apparatus forsame.

A preferred embodiment of this plant is, according to the invention,characterized in that the heat exchanger has a gas feed component whichcomprises an ejector having one or several supply apertures for powderedmaterial in the throat of the ejector, a pipe connected to the dischargeaperture of the ejector as well as a cyclone, in which the part of thepipe opposite to the ejector terminates. This embodiment of the plant isparticularly suited for carrying out the preferred embodiment of theprocess described above, in that it, with a slight loss in pressure,renders an immediate and effective dispersal of the spray-dried materialin the preheating air possible, as well as the regenerating of it fromsame.

The spray drying apparatus which is employed in the plant according tothe invention is preferably equipped with a rotating atomizer, sincesuch an atomizer renders the achieving of contact between the very hotalkali containing drying air and the aqueous slurry particles possible,which determines the separation of the alkali in the desired manner inthe form of fine particles which are carried by the gas. As the slurryhas a strongly abrasive effect on the atomizer wheel, it is expedient toemploy a wheel which is fitted with wear-resistant interchangeableinserts.

In the following, the invention will be explained in greater detail withreference to the drawing which schematically shows the preferredembodiment of the plant according to the invention as characterizedabove, as well as some of the units to which the plant, when operating,is directly connected.

1n the drawing, a feed end 1 of a rotary kiln is shown which is of alength that is suited to the production of cement according to the dryprocess. Gases issue from this end of the kiln at a temperature of theorder of magnitude of 1000-1 100 C. To the end of the kiln, an ejector 2with a throat 3 is connected. In the throat 3, there are one or severalapertures for the introduction of spray-dried slurry via the supply pipe4. The discharge aperture of the ejector is connected to a pipe 5 which,in the embodiment shown, has a perpendicular part. When spray-driedslurry is introduced into the throat 3 via the pipe 4, it is dispersedin the hot gas streaming from the kiln and conducted with same via thepipe 5, concurrently with a heat exchanging taking place between the hotgas and the relatively cold material originating from the spray dryingof the slurry. A particularly effective heat exchanging is obtained whenthe pipe, as shown, is provided with a perpendicular part, however, thepipe may also run horizontally.

The pipe 5 terminates in a cyclone 6, in which the preheated solid'material is separated from the gas. The latter is conducted from thecyclone via a pipe 7 to a ceiling air distributor 8 in a spray dryingapparatus 9. Moreover, the spray drying apparatus has a supply line 10for slurry and a rotating atomizer l l for atomizing same.

When the gas from the pipe 7, at a temperature of at least 550 C, viathe ceiling air distributor 8, passes down into the spray dryingapparatus 9, it is brought into contact with the slurry introduced viathe line 10 and atomized by the atomizer 11, which results in theappearance of a spray-dried material having a temperature of the orderof magnitude of 100-150 C, which material flows down into the pipe 4from where it is introduced in the ejector 2 as described. The gas,which hereby is cooled down to below 300 C, preferably to l7() 20()C.contains the alkali in the form ol'dispersed particles. and is led. viaa line 12, to a dust separating plzmt l3 which may possibly beconstituted of electrostatic filters.

in the embodiment shown in the drawing, the plant 13 consists of twointegral units 14 and 15, of which the unit 15 acts selectively so thatonly coarserparticles originating from the spray drying of the slurryare separated in this unit, while the fine particles and with them theprincipal amount of alkali in the gas proceed to the unit 15, in whichthey are separated and can be removed, for example, for fertilizingpurposes. From the unit 15, the gas is conducted into the atmosphere. Itis possible to employ even lower temperatures than those stated for thedischarge gas from the drier, if no electrostatic filters are used.

The coarser particles separated in the unit 14 are conducted via a pipe16, through which, by pneumatic means, they are transported to the feedend of the kiln l. The preheated material separated in the cyclone 6 islikewise conducted pneumatically to the feed end of the kiln via thepipe 16, into which it is introduced through a gate 17.

Besides the special advantage which the plant entails as a consequenceof its capacity for effective alkali removal, it possesses severaladvantages as compared to the classical wet process, in which theaqueous slurry is introduced directly into a rotary kiln. The plantaccording to the invention renders the drying and preheating section ofthe kiln superfluous, whereby the rotary kiln for a given capacity canbe constructed considerably shorter, for which reason the plant resultsin a considerable saving of investment. Moreover, the plant is, ofcourse, well suited for increasing the capacity of an existing kiln. Theplant according to the invention entails, furthermore, easier operatingconditions since it provides the possibility ofa faster control ofoperation conditions than does the long rotary kiln which is very slowto heat up.

EXAMPLE Cement slurry is worked up from sand, limestone and clay forobtaining a product which, after drying at C in a laboratory oven, hasthe composition detailed below. By burning this slurry to cement clinkersubsequent to spray drying and preheating in conformity with the processaccording to the invention, cement clinker having the composition givenin detail below is obtained.

Loss through calcination 35.2.

From the above-quoted results it appears that by means of the process aneffective alkali removal takes place since the alkali content in thecement clinker amounts to only approximately half of the value which isnormally tolerated in cement which is poor in alkali.

What we claim is:

1. A process for producing cement and eliminating harmful alkalies fromcement slurry comprising the steps of:

a. removing exhaust gas from a calcining and burning kiln;

b. preheating a spray-dried slurry by mixing said exhaust gas with saidspray-dried slurry in a heat exchanger such that the temperature of saidgas does not drop below 750 C and the temperature of said spray-driedslurry is raised to at least 550 C;

c. separating said gas from said preheated spray-dried slurd. placingsaid preheated spray-dried slurry into said kiln;

and

e producing said spray-dried slurry by spray-drying a wet 5 slurry bymixing said separated gas with said wet slurry wherein said gas whichcontains harmful alkalies is cooled to a temperature below 300 C and isexhausted.

2. The process as set forth in claim 1 wherein said gas and saidspray-dried slurry are separated in a cyclone separator and wherein theexhaust gas from said cyclone separator is at least 800 C and saidpreheated spray-dried slurry is at least 600 C.

3. The process as set forth in claim 2 wherein the temperature of saidgas after the spray-drying is below 200 C.

4. An apparatus for producing cement and removing harmful alkalies fromcement slurry comprising:

a. a kiln;

b. first exhaust means for removing exhaust gas from said kiln; heatexchanger means coupled to said first exhaust gas means for mixing saidexhaust gas with a spray-dried slurry and for preheating saidspray-dried slurry by causing a heat exchange between said exhaust gasand said slurry wherein said heat exchanger means is of the type forequalizing temperatures between a gas and a powdered material;

d. separator means coupled to said heat exchanger means for separatingsaid preheated spray-dried slurry and said exhaust gas;

e. first means coupled to said separator means for conducting saidpreheated, spray-dried slurry into said kiln;

f. spray-drying means for spray-drying a wet slurry by mixing said wetslurry with said exhaust gas separated by said separator means;

g. second means coupled between said separator means and saidspray-drying means for conducting said separated gas from said separatormeans to said spray-drying means;

h. third means coupled between said spray-drying means and said heatexchanger means for conducting said spraydried slurry from saidspray-drying means to said heat exchanger means; and

i. second exhaust means coupled to said spray-drying means for removingsaid exhaust gas which contains the harmful alkalies from saidspray-drying means after said wet slurry has been spray-dried.

5. The apparatus as set forth in claim 4 wherein said heat exchangermeans comprises:

a. an ejector coupled to said first exhaust means and said third meansfor receiving and mixing said exhaust gas and said spray-dried slurry;and

b. pipe means in which said heat exchange takes place.

6. The apparatus as set forth in claim 5 wherein said separator includesa cyclone means.

7. The apparatus as set forth in claim 4wherein said spraydrying meansincludes a rotating atomizer.

III 1F k

2. The process as set forth in claim 1 wherein said gas and saidspray-dried slurry are separated in a cyclone separator and wherein theexhaust gas from said cyclone separator is at least 800* C and saidpreheated spray-dried slurry is at least 600* C.
 3. The process as setforth in claim 2 wherein the temperature of said gas after thespray-drying is below 200* C.
 4. An apparatus for producing cement andremoving harmful alkalies from cement slurry comprising: a. a kiln; b.first exhaust means for removing exhaust gas from said kiln; c. heatexchanger means coupled to said first exhaust gas means for mixing saidexhaust gas with a spray-dried slurry and for preheating saidspray-dried slurry by causing a heat exchange between said exhaust gasand said slurry wherein said heat exchanger means is of the type forequalizing temperatures between a gas and a powdered material; d.separator means coupled to said heat exchanger means for separating saidpreheated spray-dried slurry and said exhaust gas; e. first meanscoupled to said separator means for conducting said preheated,spray-dried slurry into said kiln; f. spray-drying means forspray-drying a wet slurry by mixing said wet slurry with said exhaustgas separated by said separator means; g. second means coupled betweensaid separator means and said spray-drying means for conducting saidseparated gas from said separator means to said spray-drYing means; h.third means coupled between said spray-drying means and said heatexchanger means for conducting said spray-dried slurry from saidspray-drying means to said heat exchanger means; and i. second exhaustmeans coupled to said spray-drying means for removing said exhaust gaswhich contains the harmful alkalies from said spray-drying means aftersaid wet slurry has been spray-dried.
 5. The apparatus as set forth inclaim 4 wherein said heat exchanger means comprises: a. an ejectorcoupled to said first exhaust means and said third means for receivingand mixing said exhaust gas and said spray-dried slurry; and b. pipemeans in which said heat exchange takes place.
 6. The apparatus as setforth in claim 5 wherein said separator includes a cyclone means.
 7. Theapparatus as set forth in claim 4 wherein said spray-drying meansincludes a rotating atomizer.